Very often the same business entity shows up in more than one vocabulary.
Today you have to make a complete copy of that entity into each vocabulary and from that point on nothing will automatically keep them in sync when changes are made. Alternatively you can make one huge vocabulary that contains everything.
Neither approach is satisfactory.
It would be nice if we could do the same thing that happens in java or .net by including existing vocabulary definitions.
Then when a change is made it only has to be made in one place.
If you need to use one of these common entities but require some additional attributes all you need to do is create an entity that inherits from the main entity and you can add your own specific attributes without the need for everyone else to get them by default in the master entity.